The Han Dynasty, which ruled China from 206 B.C. to 221 A.D., fell as a result of three major factors: political instability, economic hardship and external pressure from nomadic warlords. At the root of the dynasty's problems was a growing population that strained China's resources to the breaking point.
Chinese custom dictated that each son inherited a fraction of his father's land. In an agrarian society, one in which wealth cannot be increased as the supply of land is finite, this meant that each family had to subdivide its land with each generation until not enough remained to support its people. This led to declining tax revenues and the Han government's inability to pay tribute to the warlike nomadic people on its frontiers. Without tribute, the nomads forced the Han to fight endless, costly wars on the marginal lands of its borders.
These problems were rendered unsolvable by a military and aristocratic elite that exempted itself from taxation and devoted its energy to infighting. In 189 A.D., the Emperor Shao, who was 13 years old, was assassinated by one of his generals, who then installed his preferred candidate on the throne. By 220 A.D., civil war forced the abdication of the last Han emperor, Xian, in favor of Cao Pi of Wu.